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Difficulty of proving chemical weapons

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In the NBC interview the prime minister accused Syria of using chemical weapons

The Syrian government and rebels have accused each other of using chemical weapons in the past

The Turkish government is treating around a dozen patients who have exhibited unusual symptoms suggesting they were exposed to a chemical weapons attack, a Turkish source said.

"They were not injured by any kind of conventional arms. Tests showed excessive results which produced findings to let us make that statement," a Turkish source with access to Turkish government findings told CNN, on condition of anonymity due to the sensitivity of the allegations.

The Turkish source was referring to an announcement by Turkey's prime minister which accused the Syrian government of using chemical weapons.

"It is clear the regime has used chemical weapons," said Recep Tayyip Erdogan, in an interview broadcast on NBC News Thursday night.

"There are patients who are brought to our hospitals who were wounded by these chemical weapons," Erdogan added, speaking through an interpreter.

Were chemical weapons used in Syria?

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Erdogan said he wanted the US to "assume more responsibilities and take further steps" when it comes to Syria.

The Turkish leader is expected to travel to Washington next week to hold talks with U.S. president Barack Obama.

Erdogan was once a close ally of the Syrian president Bashar al Assad. But he gradually distanced himself from Assad in 2011, as Syrian security forces mounted an increasingly deadly crackdown on anti-government protesters.

Today, Erdogan is an outspoken critic who hs repeatedly demanded Assad step down from office.

Turkey hosts more than 190,000 Syrian refugees in state-run camps, as well as many more Syrians who have fled across the border to take shelter in Turkish cities and towns.

In addition, Turkey has played a major role providing assistance and a relatively safe springboard for operations to Syrian opposition groups.

Ambulances routinely rush war-wounded across the border from Syria to Turkey for treatment in Turkish hospitals. Ankara says it has spent hundreds of millions of dollars housing, feeding and providing medical care to Syrian civilians.

The latest accusations by Turkey are not the first time allegations of chemical weapons use have emerged from Syria.

The Syrian government and rebels have accused each other of using chemical weapons on the battlefield, prompting the United Nations to call for the deployment of an international team of investigators.

This week, in an exclusive interview with CNN's Frederick Pleitgen, Syrian deputy foreign minister Faisal al-Mekdad declared his government "would never use" chemical munitions. He also accused Turkey of supply rebels with chemical weapons.

Syrian rebels as well as the Turkish government have denied such allegations, and answered with counter-accusations.

Last week, Syrian rebel spokesman Louay al Mokdad claimed opposition fighters had captured an unexploded canister from an alleged chemical weapons attack. Mokdad told CNN he hoped international experts would use the canister as evidence of alleged Syrian government war crimes.

The deadly conflict in Syria has repeatedly spilled across borders to Turkey, prompting Turkish security forces to reinforce the frontier. At Turkey's request, the NATO military alliance deployed several Patriot missile batteries to protect Turkish border cities from the threat of Syrian missile attacks.

The Turkish government appears to also be responding to more recent reports of chemical weapons use emerging from Syria.

This week, Turkey's semi-official Anatolian Agency reported that a special chemical, biological, radiation and nuclear research vehicle had been deployed to one of the busiest border gates between Turkey and opposition controlled Syria.